Neonatal whisker clipping alters intracortical, but not thalamocortical projections, in rat barrel cortex.

Retrograde axonal transport of cholera toxin B subunit (CTB) was used to compare the development of intracortical and thalamocortical connections in normal rats with those in rats in which all of the whiskers were trimmed continuously from birth. In normal animals, injections of CTB into a single barrel column resulted in an asymmetrical labeling of cells that were distributed preferentially within columns related to the same row in which the injection was placed. This anisotropy in the patterns of intracortical connections was not observed in whisker-clipped animals. In these animals, there was a significant reduction in the mean number of labeled cells in the infragranular layers, and labeled cells were distributed symmetrically around the injection site. The same injections of CTB also labeled thalamocortical neurons in the ventrobasal thalamus. Analysis of the distribution of these cells revealed that, in both control and experimental animals, the vast majority of labeled cells were restricted to a homologous (i.e., corresponding to the injected cortical barrel) thalamic barreloid. These findings indicate that manipulations of sensory experience alter patterns of intracortical, but not thalamocortical, connections.